1. Technical Field
The present invention relates to a manufacturing apparatus for a porous glass base material. Particularly, the present invention relates to a manufacturing apparatus for manufacturing a porous glass base material which allows few air bubbles or foreign materials to mix thereinto at the step of transparent vitrification, even when manufacturing a large-sized porous glass base material.
2. Related Art
VAD is a well-known method as a manufacturing method for an optical fiber base material. According to this method, a starting member is attached to a shaft which can rotatably lift up, and lifted down on this shaft into a chamber, so that glass particles generated by a core deposition burner and a clad deposition burner provided inside the chamber are deposited on the tip of the starting member and a porous glass base material (hereinafter referred to simply as porous base material) made of a core layer and a clad layer is thus manufactured.
Since the efficiency in the generated glass particles getting deposited cannot be 100%, un-deposited un-landed extra glass particles occur throughout the manufacturing process. The majority of the extra glass particles are discharged to the outside of the chamber through an exhaust outlet together with other gas such as exhaust gas.
However, some of the glass particles adhere to the ceiling or side walls of the chamber after the particles are generated by the burners until they are discharged. The glass particles attached on the inner walls of the chamber come off, fall, fly around in the chamber and adhere to a porous base material being manufactured, to be a cause of air bubbles or foreign materials mixing into the optical fiber base material when the material is subjected to transparent vitrification.
Recently, reduction in the manufacturing cost is demanded, and enlargement of optical fiber base materials is sought after. As the size of the optical fiber base materials increases, the amount of materials to be input increases, leading to the increase in the absolute amount of un-deposited glass particles even without any change in the efficiency of deposition. Thus, increase in the frequency of glass particles attached on the inner walls of the chamber coming off and falling cannot be avoided.
To solve this problem, there is proposed a method of actively introducing air into the chamber to regulate the air current in the chamber so that the extra glass particles are carried away on the regulated current and discharged to the outside of the chamber.
According to Unexamined Japanese Patent Application Publication No. 2000-109328, filters are attached on air inlets on both sides of the burners and clean air is forcibly introduced from these air inlets into the chamber to make a regulated air current.
According to Unexamined Japanese Patent Application Publication No. 2004-161506, air from a blower is supplied into the anterior chamber of the chamber and let through a filter provided inside the chamber, and the obtained clean air is supplied into the posterior chamber of the chamber to generate a regulated air current.